Detection of pulsed MeV emission from PSR B1951+32 by COMPTEL

We report on the detection at MeV energies of the radio pulsar PSR B1951+32 by the Compton telescope COMPTEL aboard the COMPTON Gamma Ray Observatory (CGRO). Folding the event arrival times with the radio ephemerides, gives for the data collected during CGRO-mission Cycles I+II a pulsar-phase distribution with two peaks, consistent in phase with the pulses measured by EGRET for energies above 100 MeV. The overall significance is>∼ 4.1σ. For Cycle III-VI data, with similar exposure, no indication was found in the phase distribution. Assuming that the source is not variable, simulations show that fluctuations in the dominating background distribution at significance levels <∼ 3σ can explain the nondetection. In addition, evidence for the presence of the pulsar in the skymaps for energies above 3 MeV is found for all Cycles of the CGRO mission. Below 3 MeV the skymaps are dominated by the strong, soft-spectrum gamma-ray source Cyg X-1, located at only ∼ 2. ◦6 from PSR B1951+32. The flux (7.7 ± 4.6) × 10−7 ph/cm2 · s · MeV measured by COMPTEL between 0.75 and 30 MeV is consistent with the measured EGRET spectrum. A single power-law fit to the combined EGRET–COMPTEL data (0.75 MeV – 30 GeV) gives an good fit with spectral index −1.89 ± 0.07. Furthermore, a break in the pulsar spectrum at MeV energies appears to be required to reconcile the COMPTEL flux with upper limits reported below 1 MeV by OSSE and RXTE.